Special Counsel Robert Mueller’s Russia investigation is looking at a controversial dossier on President Trump compiled by a former British intelligence officer, according to a report on Wednesday.

The dossier, made public in January, contains unverified, and a few scandalous, claims about Trump and his associates.

Three sources with knowledge of the probe told Reuters that Mueller’s investigators have assumed control of multiple inquiries into allegations by U.S. intelligence agencies that Russia interfered in the election to benefit Trump.

The U.S. intelligence community has said they took the allegations made in the dossier seriously.

Senate Intelligence Committee Chairman Richard Burr.

But Burr said that one witness lawmakers have been unable to question is Christopher Steele, the former British spy behind the dossier.

The committee “has hit a wall” in its requests to interview Steele.

“The committee cannot really decide the credibility of the dossier without understanding things like who paid for it, who are your sources and sub sources?” Burr said.

Burr and Sen. Mark Warner (D-Va.) said their panel has interviewed more than 100 witnesses, including Manafort and Trump son-in-law Jared Kushner, and reviewed more than 100,000 pages of documents in the past nine months.